System for controlling streamers

ABSTRACT

System for controlling cables and streamers in a seismic tow arrangement comprising at least one deflector ( 13 ) connected to one side of a tow vessel ( 10 ) via at least one wire ( 12 ), leading or the like, wherein the deflector is located in a position at a distance perpendicularly to the direction of movement ( 5 ) of the tow vessel, the vessel being equipped with a navigation system for measuring the position of the vessel. The system is characterised by comprising a command unit and a number of control units, where the control units are located on at least some of the streamers and are adapted to measure and report about their positions, that the command unit comprises means for receiving the position information from each individual control unit and calculation of possible deviations from predetermined positions, and that the system comprises means for changing the position of the streamers for re-establishing the positions of the control units.

This invention relates to a system and a method for controlling seismictowed arrangements with a towing vessel equipped with a navigationsystem for measuring the position of the vessel.

In seismic prospection at sea there is most often employed a number ofseismic cables being towed behind a vessel. The prospecting of thegeological formations in the seabed is carried out by having one or moreacoustic sources emitting sound waves downwards into the seabed wherethey are reflected at the transition between different types offormations. The reflected signals are received by sensors mounted in theseismic cables or streamers. The cables are towed along a selected pathin order to investige a given area. The movements of the vessel and theseismic tow arrangement must be controlled accurately in order to securecoverage of the desired areas.

Exact control of the positions of the vessel and the towed arrangementis particularly important when the same reflection point shall beinvestigated several times, in order to improve the measurements. Thiscan be done by controlling the time between the emitted acoustic signalsin relation to the distance between the sensors along each streamer, andthe speed of the vessel, so that the next signal is reflected from thereflection point upwards to a subsequent sensor in the direction ofmovement along the same streamer.

In order to secure accurate measurements it has been common to monitorthe position of the vessel by means of existing navigation systems, forcorrecting the position. It has been found, however, that it isdifficult to obtain a sufficient degree of accuracy, among other things,due to the dimensions and the large inertia of the vessels. Varioussystems have also been employed to compensate for errors, for example bymeans of signal processing, oversampling or by controlling thedeflectors which pull the seismic streamers laterally out from thevessel; this increases the complexity of the system, in that controlsignals and power, inter alia, must be conveyed from the vessel to thedeflectors.

U.S. Pat. No. 4,781,140 relates to a system for compensation of theorientation of the vessel in relation to the direction of movement. Theseismic streamers are connected to rigid beams the orientation of which,with respect to the vessel, is changed by means of wires when forexample the vessel is directed against the wind or the stream. Thishowever, does not involve any possibility of compensating for deviationsin the position of the vessel in relation to an intended course.

Other examples of devices and systems for keeping a seismic streamer ina correct position are found in U.S. Pat. No. 4,711,194, U.S. Pat. No.4,222,340, U.S. Pat. No. 3,961,303 and U.S. Pat. No. 3,943,483 as wellas U.S. Pat. No. 5,790,472.

The towed seismic arrangement usually comprises a number of controlunits located along each streamer, for example in order to keep thestreamer at a certain depth or position. A main problem with the knownsolutions for controlling seismic streamers is that the control of thecontrol units being mounted along each cable is not coordinated. Thismeans that the control units adjust the streamer position independentlyof one another, and this can have the consequence that the units do notreact in an optimal manner in terms of counteracting a positiondisturbance in the streamer system. Such disturbances can for example becaused by an unstable path control of the towing vessel, and turbulencefrom the towed equipment and the towing vessel.

The present invention has its main object to provide a control systemfor seismic streamers, which coordinates the effect of the control unitsso that the position regulation is at an optimum at all times, and alsowith respect to energy consumption. It is also an object of thisinvention to provide a system that can employ existing control units, sothat large and expensive reconstructions are avoided.

These objects are obtained by means of a system as stated in theattached independent claim.

The invention will be described more closely below with reference to theaccompanying drawings, illustrating the invention by way of examples.

FIG. 1 illustrates the movements of a seismic prospecting vesselaccording to prior art.

FIG. 2 illustrates the movements of a seismic prospecting vesselaccording to the invention.

FIG. 3 shows the control system for coordinated control of the controlunits according to the invention;

FIG. 4 schematically illustrates the input of information to the controlunit;

FIG. 5 schematically illustrates components of the system forcontrolling cables and streamers according to the invention.

FIG. 1 illustrates how a seismic prospecting vessel 1 according to theprior art can move under the influence of various wind and streamconditions. The vessel 1 is connected to a seismic tow arrangement 2,3,4comprising tow cables 2 and deflectors 3 which pull out the arrangementlaterally in relation to the towing direction, and seismic streamers 4comprising sensors, such as hydrophones. Besides, the towed arrangementscomprises seismic sources being connected directly to the vessel 1.

In FIG. 1 the vessel shall follow a predetermined course, but because ofwind and stream conditions, the real movement will deviate there from.The control system that compensates for the deviations will normallyresult in an oscillating movement about the predetermined course. Theseismic towed arrangement, being connected to the vessel with towingcables, lead-ins or the like 2 having fixed lengths, will follow themovements of the vessel.

In FIG. 2 the vessel 10 (compare 1, FIG. 1) shall follow a predeterminedcourse 5, but because of, inter alia, environmental disturbances (wind,stream and wave conditions) the true movements will usually deviatethere from. The control system of the vessel that compensates for theenvironmental effect or influence, usually will result in an oscillatingmovement about the predetermined course, which is illustrated by thetrue path 6 of the vessel. The seismic towed arrangement will beinfluenced by an unstable path control both in the horizontal and depthplane 7. Turbulence 8 from the deflectors and the propeller wake andturbulence 9 from the towing vessel will also give rise to positiondisturbances of the seismic streamers.

In FIG. 2 the vessel 10 according to the invention, is provided withdevices changing the length of the towing cable in response to changesin the position of the vessel in relation to the predetermined course 6.This solution is more closely described in international patentpublication No. WO 01/16623. WO 01/16623 describes a system forcontrolling seismic arrays in a marine seismic survey, comprising atleast one deflector on one side of a towing vessel coupled through awire, lead-in or similar, with the deflector being placed in a positionwith a distance perpendicular to the direction of movement of the towingvessel, and the towing vessel being equipped with a navigation systemfor measuring the position of the vessel. The vessel contains acalculation means for calculating deviations in the movement of thevessel relative to a predetermined course, based on the position of thevessel. The wire is coupled to the towing vessel through control devicesfor varying the wire length from the towing vessel to the deflector. Thecontrol devices are coupled to the calculation means for adjusting thewire length depending on said deviations in said vessel position, thusto counteract corresponding deviations in the movement of the deflector.The control devices are preferably available equipment such as winchesfor taking in or out the tow wire, suitable for connecting to thecontrol system. For control over the position of the seismic arrays, thevessel is provided with devices for measuring the positions of one ormore of the deflectors, relative to the vessel. Then the measureddeviation in the vessel's position may be compensated for by letting outor in a sufficient amount of wire until the deflector has the correctposition, both globally and relative to the vessel. For increasedprecision, known means are provided so that the heave, roll, and pitchmovements of the vessel may be measured and the control devices areadapted to compensate for these movements.

Other solutions for changing the horizontal position can be per se wellknown adjustable deflectors which can regulate the distance to thevessel by changing the lift capacity. An example of such an activedeflector is shown in international patent publication No. WO 98/25162.WO 98/25162 describes a deflector for towing after a vessel, especiallyfor use in relation to seismic surveys, being adapted to pull equipmentto a transversal position in relation to the moving direction of thevessel, comprising at least one connection point for connecting, throughwires or similar, to the vessel and the equipment to be towed. Two ormore deflector wings are adapted to provide the deflector with a liftingforce transversally in relation to the moving direction of the vessel.At least one of the wings is provided with a rotating cylinderpositioned in front of the wing adapted to increase the lift of thedeflector wing and a drive unit for driving the rotating cylinder withcorresponding devices for power supply. The rotational velocity of therotating cylinder may be controlled to adjust the lift of the deflector.

In a corresponding manner the depth can be adjusted by means of depthcontrollers as known per se, for example as described in internationalpatent publication No. WO 99/04293. WO 99/04293 describes a collapsibledepth controller for mounting in relation to seismic cables or similardevices, comprising two wings, each being rotatably connected to a mainbody relative to a first axis of rotation perpendicular to thelongitudinal direction of the depth controller, to be folded into afitted room in the main body of the depth controller. The depthcontroller comprises a first motor for rotation of an at least partiallycylindrically shaped cam plate relative to it axis, the axis essentiallycorresponding to the longitudinal axis of the depth controller, said camplate comprising at least one path or cam which along at least part ofits length has an angular extension relative to the axis of the camplate, at least one cam follower engaging into said cam plate andcomprising a first coupling device for rotatable connection to thewings, where the first coupling device is positioned at a chosendistance from the first rotational axis of the wing. Each wing is alsorotatably connected to the depth controller relative to a second axis,and the wing in a chosen distance from this axis is connected to asecond cam follower engaging in the cam plate to provide for rotation ofthe wing relative to the second axis when rotating the cam plate. Thedepth controller comprises a coaxial core being firmly mounted in thecable, and a second motor for rotation of the main body of the depthcontroller relative to the core. Rotation of the cam plate can move thesecond cam follower to lead to a rotation of the wing relative to thesecond axis, which may be used to change the angle of the wing and thusthe lift of the depth controller.

In a usual way vessel 10 is equipped with navigation systems, forexample GPS (Global Positioning System), that measures the deviations inrelation to the predetermined course, and which can be used tocompensate for deviation in the movements of the vessel. Since it issubstantially easier and quicker to pull in or pay out the towing wire,than it is to adjust the position of the vessel, the position of thetowed arrangement can be changed more quickly than the position of thevessel. Thus, the seismic towed arrangement can maintain a more stablecourse than the towing vessel 10 itself.

For exact control of the position of the seismic towed arrangement, thevessel is provided with devices for measuring the positions of one ormore parts of the towed arrangement, for example the deflectors, inrelation to the vessel. Thereby, a measured deviation of the position ofthe vessel can be compensated for directly by paying out or pulling in asufficient amount of cable, and/or by adjusting the lift capacity of thedeflectors, until the deflector has a correct position both globally andwith respect to the vessel 10. In a corresponding manner regulation ofdepth controllers will adjust the depth deviation of the streamer.

Compensation for the transverse deviation by changing the cable lengthonly, can lead to a relative displacement of the deflectors in thedirection of movement, thus giving rise to changes in the positions ofthe seismic streamers mutually. This can constitute a problem in dataacquisition, and can therefore be compensated for in other ways, forexample by regulating the length of the deployed seismic streamers or byemploying active deflectors which increase the lifting capacity so thatthe variation of the deployed streamer length will be smaller, whereasthe lifting effect of the deflectors is used to maintain a more regularposition in relation to the direction of the movement of tow vessel.

In addition, for obtaining an increased accuracy, the heave, roll andpitch movement of the vessel can be measured and compensated foraccording to the invention.

In the preferred embodiment of the invention the system comprises adeflector 13 at either side of the vessel 10, whereby the adjustmentcomprises at least the tow cables 12 to the deflectors, but can alsocomprise the other wires, streamers or lead-ins. This will also involvea possibility of adjusting the seismic streamers' position in thedirection of movement to compensate for the displacement that otherwisewould be a consequence of the length change of the tow cable 12 to thedeflector 13. FIG. 2 shows a tow situation where the seismic streamers14 are connected to one another and to the deflectors with cables whichthus results in the desired spacing of the streamers. It is of coursepossible to subdivide the towed arrangement into several parts orsections, each being provided with one or more deflectors, and where theposition of each of these parts or sections can be adjusted in order tomaintain the seismic towed arrangement at a predetermined course.

FIG. 3 shows the control system for coordinated control of the controlunits 33 according to the invention. The control system comprises acommand unit 31 which monitors each individual control unit 33 for oneor more seismic streamers 14. The control units can measure a number ofparameters, such as power or effect, depth or position, in a mannerknown per se for each of these. The control units can be located in theleading ends of the streamers, as is common today, or along eachstreamer, and can consist of conventional depth controllers “birds” withmeasuring devices, or they can be of a more sophisticated kind.

The command unit 31 comprises calculating means for calculating ofoptimum power or effect (control power) for each individual control unitin relation to a predefined position and known parameters for thecontrol unit 33. This information is processed and possible signalsregarding a change of the positions of the streamer and the controlunits, are sent to one or more of the above means or devices.

The signal transfer to and from the command unit can be via electricalor optical conductors 34 in cables or lead-ins, or can be conveyed viaradio or acoustic signals.

With reference to FIG. 4 the command unit 31 receives signals from thecontrol units 33 based on measurement methods known per se, in order tomaintain depth, effect or power, and position. For example can depth bemeasured by pressure measurement, power or effect by means ofmeasurements of fin or wing angle of a sensor, and position by means ofa compass or via acoustic measurement methods in order to determinerelative positions between the cables or streamers. Additionally alsoposition measurements and possibly other measurements can be performedfrom instruments 34 mounted in the streamer itself.

Besides, as mentioned above, the tow vessel comprises navigation aids 32being connected to the command unit 31 in order to supply informationthereto, for example regarding global position (via satellite navigationsystems or the like), movement, gyro and the like, so that the positionsof the streamers with respect to the movements of the vessel and thegeological formations to be investigated, can be controlled.

The actual control can take place by having the planned movement of theseismic tow arrangement in the form of position, the depth of thestreamers and so forth stored in the command unit, and informationregarding the real positions thereof being calculated on the basis ofthe global position of the vessel and the measurements from theindividual control units. Deviations from the desired positions arerecorded and the different control devices, as for example activedeflectors, winches, depth controllers and the like, are activatedindividually in relation to the desired positions of the various partsor sections of the tow arrangement.

FIG. 5 schematically illustrates components of the system forcontrolling cables and streamers according to the invention. The vessel21 tows streamers 24 by means of tow wires 22. At least one control unit33 (see FIGS. 3 and 4) comprises a depth measuring instrument. A depthcontroller 25 is connected to a streamer 24 for adjusting the verticalposition of the streamer 24. A control device 26 is adapted to vary thetow wire 22 length from the tow vessel 21 to the deflector 23. Thedeflector 23 contains means 27 for adjusting the lifting capacity of thedeflector 23 for adjusting the position of the deflector 23.

Although the figures of drawings show a symmetrical tow arrangement, itis of course possible to take advantage of the invention in the case ofasymmetric tow arrangements, for example in the case of several vesselstogether forming a seismic tow arrangement. A solution wherein the wholetow arrangement is located at one side of the vessel, can also becontemplated, for example in prospecting near land, where the vesselmust be kept at a certain distance from the shore.

Although the invention is primarily described here for the purpose ofcompensating for drift with respect to a predetermined course, it isobvious that it can also be employed for keeping the seismic towarrangement along a predetermined path, whereas the vessel for examplemakes manoeuvres in order to avoid obstacles. For example there may bethe case of a river where the possible sailing path is not necessarilystraight.

1. A system for controlling cables and streamers in a seismic towarrangement comprising at least one deflector (13) connected to one sideof a tow vessel (10) by at least one tow wire (12), wherein thedeflector is located at a position with a distance perpendicular to thedirection of movement (5) of the tow vessel, the vessel being equippedwith a navigation system (32) for measuring the position of the vessel,a command unit (31) and a plurality of control units (33), the controlunits being located on at least some of the streamers (14) and beingadapted to measure and report their own positions, wherein the commandunit (31) comprises means for calculating deviation in the position ofthe tow vessel; means for receiving position information from eachcontrol unit (33); and means for calculating deviations frompredetermined positions of the control units, based upon the receivedposition information from the control units, and the system comprisesmeans for changing the position of the streamers in order to restore thepositions of the control units, to maintain the streamer at a givendepth and position, including control devices for varying the length ofthe tow wire (12), to vary the transverse position of the deflector (13)with respect to the vessel (10), based upon the received positioninformation from the control units and the calculated deviation in theposition of the tow vessel, and means for coordinating the control unitsin changing the position of the streamers.
 2. The system according toclaim 1, wherein the position information from at least one control unit(33) comprises horizontal and vertical position.
 3. The system accordingto claim 1, wherein the command unit (31) comprises means for measuringthe relative position of the deflector (13) with respect to the vessel(10), and adjustment comprises control of the global position of thedeflector based on the position of the tow vessel and the relativeposition of the deflector with respect to the vessel.
 4. The systemaccording to claim 1, wherein at least one control unit (33) comprises adepth measuring instrument and the means for changing the position ofthe streamers comprises at least one depth controller, for adjusting thevertical position of the streamers.
 5. The system according to claim 1,wherein, the tow wire (12) is connected to the tow vessel (10) viacontrol devices-adapted to vary the tow wire length from the tow vesselto the deflector (13) and the control devices are in contact with themeans for calculating deviation, to adjust the wire length on the basisof said deviation in the position of the tow vessel, in order tocounteract corresponding deviations in the movement of the deflector. 6.The system according to claim 5, wherein the control device adapted tovary the tow wire length from the tow vessel (10) to the deflector (13)is a winch.
 7. The system according to claim 1, wherein the command unit(31) also comprises means for measuring deviations in the orientation ofthe vessel.
 8. The system according to claim 7, wherein the measureddeviations in the orientation of the vessel are roll, heave and pitch.9. The system according to claim 1, further comprising means foradjusting the lifting capacity of the deflector (13) for adjusting theposition of the deflector.
 10. The system according to claim 1, whereinthe streamer (14) comprises means (33) for measuring the position of thestreamer, and means for transmitting the position of the streamer to thecommand unit.
 11. The system according to claim 1, wherein the controlunits (33) are located at selected points on the streamers (14), thedeflectors (13) and the depth controllers, for measuring selectedvariables at each control unit.
 12. The system according to claim 11,wherein the measured selected variables are lifting capacity, positionand depth.
 13. The system according to claim 1, wherein the tow wire(12) by which at least one deflector (13) is connected to one side ofthe tow vessel (10) is a lead-in wire.